The present invention relates to a method of charging several electrochemical cells, particularly Lithium-Polymer cells, connected in series, a charging means for adjusting the voltage and charging current.
The invention is applicable in particular to the use of accumulator systems for powering electric vehicles (2, 3 and 4 wheels) and thermal vehicles with self-contained electric mode, where it is suitable for optimising the charging of cells so as to increase the range of action and self-sufficiency of the vehicle but also to reduce the charging time, with a view to increasing the availability of the vehicle.
According to one known method, the state of charge of each cell is detected permanently, and when one cell reaches a predetermined threshold voltage, the current supplying the cell is shunted.
A system of electrochemical accumulators (called a battery) for an electric vehicle is composed of sub-assembly elements connected permanently in series, called “cells”, if their unit voltage is between 1V and 4V according to the technology of the accumulator, or “modules” if the voltage of the assembled series of cells is between 6V and 50V.
A charging device (external to the battery) is used for re-charging the battery. It is connected to an electrical distribution system, and arranged with a communication system for each module. This communication system allows charging of the battery (rules and signals) to be controlled.
The known method of charging the battery is carried out as a general rule, in two stages:
The first stage consists of charging the battery as quickly as possible, either at a constant current (Ia) or at constant power (Wa). This charging is completed when the battery reaches a predetermined upper threshold voltage.
The second stage consists of finishing charging either at a reduced constant current (I) or a reduced constant power (W). A modification of the second stage of this charging method consists in charging the battery at a constant potential (U), leaving the charging current to decrease to a minimum threshold current.
This method has the disadvantage that the state of charge of the battery can only be controlled globally. It does not allow maximisation of the stage of charge of all the cells without overcharging some of them.
In order to optimise the charging of the cells and the modules which make up the battery, a known method has been proposed of complete shunting of the charging current of each cell. Its function is to allow charging of the undercharged cells without overcharging the cells which have already been charged. The charging of the cells is in this case individualised.
The implementation of devices for carrying out a method of complete shunting of the charging current of each cell or of each module results in the first place, in an excess cost connected with the use of power and control electronic components, and in the second place, in excess weight, because of the presence of a liquid cooling system.